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    <script>
    // 3.声明一个函数，传入一个纯数字的数组作为形参。返回该数组中元素的平均值
    function arrAll(arr){
        let sum = 0
        for(let i = 0; i < arr.length; i++){
            sum += arr[i]
        }

        return sum / arr.length
    }

    console.log(arrAll([1,2,3,4,5,6,7,8,9]));

    </script>
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